1. Technical Field
The invention relates to carburetors for internal combustion engines, of the type provided with an automatic starting device comprising a choke valve whose gradual opening, during heating up of the engine, is controlled by an element whose temperature increases progressively as the engine heats up, such as, for example, a bimetallic spiral subjected to the temperature of the cooling water of the engine or a capsule containing a heat-expandable material.
The invention relates more particularly to carburetors which comprise successively, in an intake duct or induction passage, from downstream to upstream, an operator operable throttle member, an opening of a main fuel delivery system and a choke valve urged toward closure by the element sensitive to temperature as long as said temperature is below a limit value and toward opening by the air flow round it.
2. Prior art
Most prior art carburetors of this type further include a pneumatic element subject to the vacuum which exists in the intake duct downstream of the throttle member and arranged to give the choke, as soon as the engine is self operative, a minimum degree of opening.
In carburetors provided with such a pneumatic element, the considerable degree of vacuum which exists in the intake duct at the mouth of the main delivery system when the choke is closed, enables a rich air/fuel mixture to be obtained during the time that the engine is driven by the starter motor; the choke opens partially as soon as the engine is self running, which avoids flooding and stalling of the engine due to a fuel excess.
Existing devices of this type have a serious drawback. If the engine is started up when completely cold, for example, not having run for several hours, and then is stopped after a short time (for example, after a period of operation of from one to three minutes), it is difficult to start up the engine again immediately: Since the cooling water of the engine has not had time to heat up notably, the choke valve closes again completely as soon as the engine stops. Consequently, the engine is again supplied with a mixture having a high fuel/air ratio on a further attempt to crank it into operation. Since the engine which has just run, does not need a richness as high as that of an engine which has remained inoperative for several hours, the excess of richness makes starting up difficult.